1. Field of the Invention
The present invention relates to a channel allocation method in a CDMA (Code Division Multiple Access) mobile communication system and, more particularly, to a downlink control channel allocation method suitable for using a physical shared channel as a control channel.
2. Description of the Background Art
In general, a downlink channel in a CDMA IMT-2000 system employs a method of multiplying an orthogonal code such as an orthogonal variable spreading factor (OVSF) or Walsh code to a transmission bit to spread it.
Lately, studies are being conducted to determine a transmission method of a new data dedicated channel in order to increase a transmission rate of uplink in the IMT-2000 system. In case of applying the enhanced data dedicated channel to uplink, a downlink control channel has the following characteristics.
First, it is assumed that new signaling information (control signals) required for operating an enhanced dedicated channel (E-DCH) is required for downlink. In this case, the information can be considered in association with an automatic repeat request (ARQ), scheduling and some other cases. Even though signal information requires high reliability, such signaling information can be multiplexed through a shared channel and transmitted to multiple terminals thanks to its small amount of data. Requirement for a physical channel structure for an L1 signaling at downlink is as follows.                Preferably, the L1 signaling on downlink exists independently from a data dedicated channel of downlink.        Signaling of every user should be transmitted at every transmission time interval (TTI).        In order to reduce a delay phenomenon, signaling should be detected by a terminal before deinterleaving.        
Existing methods proposed for the downlink signaling are as follows.
A first method is to transmit signal information by using an existing downlink data/control channel. Namely, in this method, the size of signaling information is not so big that signaling information can be transmitted by changing a frame structure of a downlink control channel or signaling information can be transmitted by using a remaining space after a downlink data channel is punctured. In addition, in this method, signaling information can be also transmitted by increasing a transmission rate of the downlink data channel.
A second method is to use an independent orthogonal code channel. In this method, signaling information is transmitted to each terminal by using an independent orthogonal code channel. In this case, an independent physical channel can be used for transmission of a downlink L1 signaling. The physical channel can be used as a dedicated channel of each terminal or as a shared channel for multiple terminals.
The method of using the independent orthogonal code channel can be divided into a method of using a dedicated channel and a method of using a shared channel. In the method for transmitting signaling information by using the dedicated channel, an independent orthogonal code channel is allocated to each terminal and signaling information is transmitted thereto.
The method of using the shared channel is divided into a method of using time-division shared channel and a symbol-level orthogonal code division multiplexing method.
The method of using a time-division shared channel is to time-divide one orthogonal channel and allocates it to multiple users. Namely, one TTI is divided by the number of users and the divided time interval is allocated to each user to transmit signal information. At this time, the number of bits differs depending on the size of requested signaling information.
The symbol-level orthogonal code division multiplexing method is to transmit signaling information by using an orthogonal signal in units of symbol. Namely, in general, in the CDMA downlink, the orthogonal code channel uses a orthogonal signal in units of chip, and in order to extend the orthogonal code channel, signaling information is transmitted by using the symbol-level orthogonal signal. In this case, the orthogonal signal can be generally extended by using a Hadamard code, and data is transmitted by using the Hadamard code which is able to support the number of a maximum transmission users.
However, considering the downlink physical channel structure required for the case where the data dedicated channel is applied to uplink, the related art signaling transmitting method has the following problems.
1. The Method of Using Existing Downlink Data/control Channel
This method has such problems that an existing channel structure must be changed, a quality of the existing channel can be degraded in case of using the data channel, and it is difficult to secure a sufficient transmission space for transmitting signaling information.
2. The Method of Using the Independent Orthogonal Code Channel
1) Use of the Dedicated Channel
In spite of the flexible and easy channel allocation and operation, the method of using the dedicated channel has the following problems: Because the orthogonal channel of downlink is a critical resource, there is a high possibility that it can not be used sufficiently. In addition, as the number of codes increase, a PAR (Peak to Average Ratio) also increases considerably.
2) Use of the Shared Channel
In Case of Using Time-Division Multiplexing
The time-division multiplexing method in which time is divided has advantages that the PAR does not increase and it can be easily implemented. However, it is disadvantageous in that it is difficult to flexibly allocated a signaling channel according to change in the number of data, and thus, more signaling information is required. In addition, it is difficult to flexibly allocated power according to a channel situation by users (it is difficult to effectively use resources).
In Case of Using Symbol-Level Code-Division Multiplexing
The symbol-level code division multiplexing has an advantage in that because it does not have a time delay, users can transmits signaling information simultaneously, but if there are too many users, the length of the orthogonal code should be lengthened. In addition, the more the number of users or the transmission rate increases, the more orthogonal codes should be used, causing increase in the PAR.
As for the orthogonal code, in order to maintain orthogonality, a channel must not be changed during one period, but if the length of the code is lengthened, the orthogonality may be broken. In addition, if the length of the code is continuously increased to increase available users, a problem arises that the length of the code would exceed a transmission unit.
Moreover, in order to increase the transmission rate, a plurality of orthogonal codes should be allocated to one user. Then, one user is to receive several orthogonal code channels, complexity of a terminal increases.